To determine the purity of a sample of Na2S2O3, a sample is titrated coulometrically using I– as a mediator and I3– as the titrant. A sample weighing 0.1342 g is transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL portion is transferred to an electrochemical cell along with 25 mL of 1 M KI, 75 mL of a pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at a constant current of 36.45 mA requires 221.8 s to reach the starch indicator endpoint. Determine the sample’s purity.

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The coulometric titration of S2O32 with I3 is 2S2O32aq 3I3aq S4O62aq 3Iaq ... View the full answer